Ventilation fans have been widely used in the industry, especially in the home appliance business. For example, small cooling fans of the propeller type and the centrifugal type are used for the purpose of cooling and air circulation or ventilation in electric appliances and office automated machines.
As it is well known, the ventilation fan includes an apparatus provided with a number of rotary blades which continuously transfer kinetic energy to air or vapor and raise the pressure of air, etc. There is now the tendency to pursue designs with higher performance and lower noise levels.
In light of these points, there are axial fans designed for realizing low noise as described in Korean Patent Publication No. 88-521 and its corresponding Japanese Patent Publication No. 90-26079. The axial fan includes an axial impeller determining the entire shape thereof by defining the three dimensional distribution with respect to each of the blades' load. In other words, the axial fan is usually constructed to have four or more blades evenly spaced apart from one another (one such blade being shown in FIG. 1), but the blades are arranged in an irregular spacing for the purpose of damping the specific frequency of noises such as Blade Passage Frequency(BPF) component. Also, the axial fan includes blades 1 deposited on a plane disposed at a right angle to a rotating shaft 3. In the coordinate system with the suction direction of an air flow being considered as a positive direction, point P.sub.R is assumed as the center point of the blade line when the blade 1 is cut by a cylindrical surface of radius Rb, while point Pb is assumed as the center point of the blade chord in a cross section when the boss part 2 of the blade is cut by the cylinder surface of radius Rb. And, it is assumed that a distance Ls is formed between the center point P.sub.R of the blade chord Pb and a plane Sc. The center point of the blade chord is continuously positioned in the positive direction for the plane Sc, thereby obtaining an angle of .delta.Z which can be expressed by .delta.Z=tan.sup.-1 {Ls/(R-Rb)} as .delta.Z=12.5.degree.-32.5.degree.. On the other hand, since the blade is projected on a plane disposed at a right angle to the rotary shaft 3, Pb' is assumed as the center point of the blade chord in the cross section, when the boss part 2 is cut by the cylindrical surface of radius Rb. The coordination system is formed to have a straight line connecting a point O with the point Pb' as an X-axis, with a rotary shaft serving as an original point O. At that time, if the blade is cut by the cylinder surface of radius R, then, Pb' is assumed as the center point of the blade chord, and an angle .delta..theta. formed by the straight line P.sub.R '-0 and the X axis is defined as .delta..theta.=.delta..theta.t.sub.x (R-Rb)/(Rt-Rb), wherein R.sub.t is a radius of the blade edge, Rb is a radius of the blade boss, .delta..theta.t is an angle formed by the straight line Pt'-0 and the x-axis, and .delta..theta.t=40.degree.-50.degree.. In addition, when the axial fan including the blade 1 is assembled into a bell-mouth, as shown in FIG. 2, the blade defines a plane crossing at a right angle with respect to the rotary shaft 3, from which is produced a curved surface of radius BR. The bell-mouth defines a duct having a straight portion ld and an internal diameter DB accommodating an external diameter DT of the fan blade. In the bell-mouth, when there is assumed a distance 1x between a trailing edge of the periphery of the blade and the final end of the duct, a relation where BR=0.08 DT-0.2 DT, DB=1.01 DT-0.01 DT, ld=1.04-0.1 DT and lx=0-0.04 DT is obtained.
Accordingly, the flow efficiency and the noise level are determined according to the non-dimensional interval between the bell-mouth and the axial fan, the curvature radius BR of the bell-mouth, and the relative position lx between the fan and the bell-mouth. A characteristic curve demonstrating the incremental changes of the static pressure generated by the fan in relation to the change of the flow rate is shown in FIG. 5. In other words, as the load applied to the fan becomes larger, the flow rate is reduced. If the load is reduced, the increment of the static pressure is reduced and the flow rate is reduced. As shown in FIG. 4, when the cross section of blade 1 is projected in the periphery direction of the fan, assuming that the blade is cut by the cylindrical surface of radius R, the section is developed to a two dimensional plane, obtaining developed figure. In the developed figure. A camber line is formed as a circular arc and if .theta. is assumed for the center angle, a distribution of .theta. in the radial direction is given as .theta.(.theta..sub.t -.theta..sub.b).times.{{(R-Rb)}/(Rt -Rb)}}+.theta..sub.b, wherein .theta..sub.t is a camber angle of the blade edge, .theta..sub.b is the camber angle of the blade boss, .theta..sub.t is 20.degree.-30.degree., .theta..sub.b is 27.degree.-37.degree., .theta..sub.t &lt;.theta..sub.b. Also, an angle .xi. is formed by the blade chord and the straight line in parallel to the rotary shaft passing through the leading edge of the blade. Then, the radius direction distribution is given by .xi.=(.xi.t -.xi.b).times.(R-Rb)/(Rt-Rb)+.xi.b, wherein .xi.t is a staggering angle at the blade edge, .xi.b is the staggering angle ato the blade boss, .xi.t is 62.degree.-72.degree., .xi.b is 53.degree.-63.degree. and .xi.t&gt;.xi.b. Therefore, if the fan flow rate Q is decreased, the speed component Ux in the axis direction is reduced. An angle of attack between the blade chord and the speed vector is increased, while the lift force is increased, and the static pressure is increased. On the contrary, if the fan flow rate is increased, the angle of attack is reduced and the pressure difference between the fan pressure edge and the absorbing edge is reduced, whereby the lift force is reduced.
Herein, it is noted that the coventional axial flow fan does not cause the asymptotic decay which reduces the load to be applied to the fan like the performance curve of an ordinary small-sized propeller as shown in FIG. 5, but it reduces the flow rate at before and after the normal operation point of the fan and causes the S-hysteresis which decreases the pressure in the performance characteristics. Namely, before and after the normal operation of the fan, the flow rate is changed, thereby producing a bad effect on the cooling performance of an entire system.
Furthermore, the noise level must be reduced near where the air pressure is sharply dropped on the noise spectrum of each load as shown in FIG. 6, but due to the leading edge separation on the pressure surface of a blade the narrow band noise is increased. Consequently, noise is caused near the leading edge of the axial fan due to the erroneously mistaken plate design of the blade section, which results from the S-hysteresis preformance characteristics. In other words, when the small propeller fan is constructed to have a plurality of blades, the blade is designed to be a thin plate in order to minimize the rotary inertia and thus reduce the electric power consumption. Also, as the flow rate is increased, the bell-mouth is positioned at a zone 1, but at that time the pressure at the leading edge is reduced, and also the flow rate is decreased. That is because the leading edge of the blade is designed to be parallel to a plane which is positioned perpendicular to the direction of air inflowing followed by the leading edge flow separation.
Accordingly, the invention is supposed to change the position angle of a blade, thereby improving the unsteady state for the flow rate.